1. Field of the Invention
The invention relates to an operating device for a friction clutch arranged in a housing bell between an internal combustion engine and a transmission in the drive train of a motor vehicle, especially a commercial vehicle. Such an operating device includes a clutch release bearing arrangement that can be moved substantially coaxially to the friction clutch for the purpose of operating the friction clutch, and a positioning servo arrangement with a pressure medium power cylinder arrangement that acts upon the clutch release bearing arrangement and can be operated, via a control valve connected to a pressure medium source, in dependence on a control variable, which represents a target position, and a current variable that represents the axial position of the clutch release bearing arrangement.
2. Discussion of the Prior Art
An operating device of this type is known, for example, from DE 33 21 578 C2. The known operating device has a positioning servo arrangement in the form of a low pressure servo power amplifier. The servo power amplifier, designed as a low pressure brake power amplifier, is embodied integrally with a pneumatic power cylinder and a control valve, and is arranged outside of the housing bell. Two working chambers of the pneumatic power cylinder are separated from each other by an axially movable piston and an elastic diaphragm. The first working chamber, designed as a low pressure chamber, is attached to an intake line system of the internal combustion engine. The other working chamber, serving as a control chamber, can be connected by means of the control valve either to the low pressure chamber or, via a pressure compensation opening, to the atmosphere. The low pressure servo power amplifier is operated via a control rod, which hits against a valve body of the control valve and can be moved axially via a cam driven by means of an electric motor. An axial movement of the control rod activates the control valve so that the piston follows the movement of the control rod in a power-amplified manner. The movement of the piston, via a hydraulic input cylinder, acts on an output cylinder that is connected to the input cylinder and is arranged outside the housing bell. The output cylinder, in turn, acts on a disengaging fork associated with the clutch release bearing arrangement.
Furthermore, an operating device of the aforementioned type is known that has a pneumatic power cylinder as the pressure medium power cylinder arrangement. The operating device is attached externally to the housing bell in the form of an integral unit that comprises the pneumatic power cylinder, a hydraulic output cylinder and the control valve. The piston of the pneumatic power cylinder is located on a rod element that forms the piston of the hydraulic output cylinder. The rod element is connected to a tappet that extends into the interior of the housing bell and acts upon a disengaging fork associated with the clutch release bearing arrangement. Attached to the hydraulic output cylinder are an input cylinder, which is operable by a clutch pedal, and a control input of the control valve. The control valve, depending on a hydraulic pressure applied at the control input, controls the supply of pressurized air to the pneumatic power cylinder or the release of air from the pneumatic power cylinder so that a predetermined hydraulic pressure determined by a compression spring arrangement of the control valve is established at the control input. The hydraulic output cylinder serves as a measurement cylinder which detects the position of the rod element and thus indirectly detects the position of the clutch release bearing arrangement. During operation of the input cylinder, forces are exercised directly on the rod element and thus on the clutch release bearing arrangement via the hydraulic output cylinder serving as a measurement cylinder, in addition to the operating forces of the pneumatic power cylinder resulting from the supply of pressurized air to the power cylinder.
The idea has arisen of marketing an operating device of the aforementioned type, in which the pressure medium power cylinder arrangement is located inside the housing bell. In such an arrangement, in principle, a control valve of the type described above can be used with a hydraulic output cylinder that serves as the measurement cylinder and, upon activation of the control valve, exercises forces on the clutch release bearing arrangement. However, this idea is problematic, insofar as centrical pickup of the disengagement signal on the power cylinder arrangement is impossible, at least when the power cylinder arrangement, as preferred, has, as the pressure medium main cylinder, a pressure medium ring cylinder that encompasses the clutch axis. The disengagement signal must thus generally be picked up extra-centrically on the pressure medium power cylinder arrangement or, in some cases, on the clutch release bearing arrangement, which leads to the occurrence of lateral forces or tilting moments. Such lateral forces or tilting moments are extremely problematic, because they can lead to self-locking of the pressure medium power cylinder arrangement, so that the functional reliability of the operating device is no longer ensured under all circumstances.